1. Field of the Invention
The present invention relates to spacecraft operation and, more particularly, to controlling the formation of a constellation of spacecraft.
2. Prior Art
Worldwide satellite communication systems, such as for example Globalstar(trademark), and Iridium(trademark), employ a large constellation of communication spacecraft or satellites. For example, Globalstar(trademark) generally uses a constellation of about 48 spacecraft in low earth orbit (LEO). The Iridium(trademark) constellation has 66 spacecraft. The Global Positioning System (GPS) also employs a constellation of spacecraft to provide positioning services. To provide the commercially desirable levels of coverage, the satellites within these and other constellations of spacecraft are maintained in a predetermined constellation formation. Conventional methods of formation keeping in a constellation of spacecraft have generally relied on one of two approaches or a combination of these approaches. In the first conventional approach to maintain constellation formation, each of the spacecraft in the spacecraft constellation has GPS receivers. The GPS receivers aboard each spacecraft provide high precision orbital data for each spacecraft. This orbital data for each spacecraft may be transmitted (i.e. downlinked) to a ground based processing facility (e.g. ground control station) which determines the high precision orbit solution for each spacecraft in the constellation. The ground based processing facility examines the orbit solution of each spacecraft and commands maneuvers for each spacecraft to maintain constellation formation in response to observed or anticipated orbit deviations or bias of each spacecraft. The other conventional approach for maintaining constellation formation determines the orbit solution for each spacecraft in the constellation using ground based measurements (e.g. tracking using ground antennas that obtain spacecraft position information relative to the ground antennas locations). The ground based measurements for each spacecraft are again sent to a ground based processing facility which examines the orbit solution for each spacecraft, and sends maneuver commands to each spacecraft to maintain the constellation formation. Globalstar(trademark) uses on board GPS navigation equipment carried by each spacecraft in the constellation. Data gathered by this equipment is downlinked to a ground facility for processing. A ground based approach for constellation formation keeping is used by the Iridium(trademark) and GPS systems. Both conventional approaches treat constellation formation keeping substantially the same as maintaining the orbit of an individual spacecraft for each of the spacecraft in the constellation. Accordingly, both conventional approaches for constellation formation keeping are inefficient, and costly. This is due to the duplication in the equipment (e.g. GPS receivers on all spacecraft, or extensive number of ground based tracking stations) used to identify the orbit solutions for each spacecraft, and of the processing cost for substantially simultaneously examining the orbit solutions of all spacecraft in the constellation. The present invention overcomes the problems of the prior art as will be described in greater detail below.
In accordance with a first method of the present invention, a method for controlling a spacecraft is provided. The method comprises the steps of providing a first spacecraft in a known predetermined orbit, and a second spacecraft in a second predetermined orbit, measuring a distance between the spacecraft, using the measured distance for determining an orbital error bias, and maneuvering one of the spacecraft to compensate for the orbital error bias. The measured distance is used for determining the orbital error bias of the second spacecraft relative to the second predetermined orbit. The second spacecraft is maneuvered to compensate for its orbital error bias and to maintain the second spacecraft in the second predetermined orbit.
In accordance with a second method of the present invention, a method for controlling a spacecraft constellation is provided. The method comprises the steps of providing a first spacecraft of the spacecraft constellation in a first predetermined orbit, a second spacecraft of the spacecraft constellation in a second predetermined orbit, and a third spacecraft of the spacecraft constellation in a third predetermined orbit. A first distance is measured between the second spacecraft and the first spacecraft. A second distance is measured between the third spacecraft and the second spacecraft. The first measured distance is used for determining an orbital error bias of the second spacecraft relative to the second predetermined orbit. The second measured distance and orbital error bias of the second spacecraft are used for determining a orbital error bias of the third spacecraft relative to the third predetermined orbit. When resources are available, the second spacecraft is maneuvered to compensate for its orbital error bias and to maintain the second spacecraft in the second predetermined orbit. The third spacecraft is maneuvered to compensate for its orbital error bias and to maintain the third spacecraft in the third predetermined orbit.
In accordance with a first embodiment of the present invention, a spacecraft is provided. The spacecraft comprises a spacecraft bus, a range finder, and a controller. The spacecraft bus has a maneuvering system mounted thereon. The range finder is connected to the spacecraft bus for measuring a distance between the spacecraft and another spacecraft. The controller is connected to the spacecraft bus. The controller is communicably connected to the range finder for receiving the distance measurement from the range finder. The controller is programmed for determining an orbital error bias of the spacecraft from the distance measurement. The controller is further programmed for operating the maneuvering system in response to the determined orbital error bias.
In accordance with a second embodiment of the present invention, a constellation of spacecraft is provided. The constellation of spacecraft comprises a first spacecraft, and a second spacecraft. The first spacecraft is in a first predetermined orbit. The second spacecraft is in a second predetermined orbit. The second spacecraft orbits generally in formation with the first spacecraft in the first orbit. The second spacecraft has a range finder for measuring a distance between the second spacecraft and the first spacecraft. The second spacecraft has a controller communicably connected to the range finder for receiving the measured distance from the range finder. The controller has programming for determining an orbital error bias of the second spacecraft from the measured distance. The controller includes programming for operating a maneuvering system of the second spacecraft in response to the determined orbital error bias for maintaining the second spacecraft in the second predetermined orbit generally in formation with the first spacecraft in the first predetermined orbit.